The present invention relates to microfibers of syndiotactic vinyl aromatic polymers and nonwoven mats of the microfibers particularly useful in the field of filtration and insulation. The present invention also relates to a melt-blowing process for the production of the microfibers and the nonwoven mats.
Various melt-blowing processes for producing nonwoven mats or webs of microfibers have been described heretofore in patents and literature.
U.S. Pat. No. 2,411,660 describes a melt-blowing process for the manufacture of nonwoven fabrics from plastics for abrading, scouring, filtering, etc. U.S. Pat. No. 3,849,241 discloses a process for producing a melt-blown nonwoven mat wherein a fiber-forming thermoplastic polymer resin having a specific initial intrinsic viscosity is subjected to degradation in the presence of a free radical source compound. Several melt-blowing processes for the production of a nonwoven thermoplastic fabric or a composite thereof are taught in U.S. Pat. Nos. 4,041,203, 4,196,245 and 4,302,495. R. L. Shambaugh discussed several factors of a melt-blowing process using dimensional analysis in "A Macroscopic View of the Melt-Blowing Process for Producing Microfibers", Ind. Eng. Chem. Res., Vol. 27, No. 12, 2363-72 (1988).
On the other hand, syndiotactic polymers of vinyl aromatic monomers have recently been developed. U.S. Pat. No. 4,680,353 discloses a polymerization of syndiotactic polystyrene using certain titanium based Kaminsky-Sinn catalysts. In U.S. Pat. No. 4,774,301 a similar process employing a zirconium containing Kaminsky-Sinn catalyst is disclosed. In EP's 271,874, 271,875 and 272,584 further description of suitable Kaminsky-Sinn catalysts is provided. U.S. patent application No. 223,474 filed July 22, 1988 and EP 291,915 teach a process for producing fibers of syndiotactic polystyrene using a melt-spinning process which clearly differs from the melt-blowing process.
The aforementioned patents regarding a melt-blowing process indicate that a broad range of plastic materials may be used for producing nonwoven mats of microfibers U.S. Pat. No. 2,411,660 states that a great variety of plastics may be used, such as vinylidene chloride, polystyrene, polyphenylenesulphide, polyvinyl alcohol, polyvinyl acetate, methyl methacrylate, polymeric amide, copolymer of vinyl chloride and vinyl acetate, latex compositions, cellulosic and petroleum derivatives, protein-base materials, glass, etc. U.S. Pat. No. 4,041,203 describes that among the many useful thermoplastic polymers, polyolefins such as polypropylene and polyethylene, polyamides, polyesters such as polyethylene terephthalate, and thermoplastic elastomers such as polyurethanes are anticipated to find the most wide spread use in the preparation of the materials described herein (nonwoven thermoplastic fabrics of microfibers). However, it has been discovered that certain polymers, particularly certain crystalline polymers, are difficult to melt-blow. For example, it is found that crystalline polyamide is not suitable for melt-blowing because of a lack of suitable melt viscosity and melt elasticity properties. If a melt-blowing process is carried out at high temperature at which the crystalline polyamide can be processed, the thermal degradation of the molten polymer will readily occur. In addition suitable conditions of extrusion rate and air velocity cannot be attained to avoid the twin problems of fiber attenuation and breakage or slub formation, i.e., formation of globular agglomerates of polymer.
Currently, filters comprising fibers of polytetrafluoroethylene, polyester, polyimide or glass are used in high temperature filtration of corrosive media such as acids, alkali, chlorine cell effluent, flue gas, etc. However, nearly all of the existing materials have proven unsatisfactory for extremely demanding, high temperature filtration applications. In particular, filtration media comprising the polyester fibers lack sufficient hydrolytic stability and chemical resistance under actual operating conditions, and glass fibers are readily attacked by alkali.
It would be desirable if there were provided a microfiber and a nonwoven mat, fabric, web, or similar structure prepared therefrom comprising a vinyl aromatic polymer having a high degree of syndiotacticity and crystalline structure, which have good hydrolytic stability, good chemical resistance and good high temperature resistance.
It would also be desirable if there were provided a melt-blowing process for producing a fiber, preferably a microfiber or a nonwoven mat therefrom, comprising a vinyl aromatic polymer having a high degree of syndiotacticity and crystalline structure.